Import Oslo's common rootwrap to Neutron

Use the common oslo-incubator rootwrap rather than maintain a
specific fork within Neutron.

- Migrated DnsmasqFilter use in dhcp.filters to the new EnvFilter
- Changed environment passing in ip_lib's netns.execute so that
  it can be properly matched using IpNetNsExecFilter + EnvFilter.
  It now calls "ip netns exec ns env A=B C=D command" instead of
  "A=B C=D ip netns exec ns command". Adjusted tests accordingly.

All the other changes are coming directly from the Oslo "rootwrap"
module sync.

Notes:
- Neutron locates its rootwrap.conf in etc/ rather than in etc/neutron
- Neutron maintains a specific bin/quantum-rootwrap-xen-dom0 which
  requires additional config in rootwrap.conf

Both behaviors were preserved in this commit, but this may need to be
addressed in the future to simplify future oslo-rootwrap updates.

Implements bp: quantum-common-rootwrap

Change-Id: I02879942a9d1169a71aa4d684c1b9ec109a6de32
Merge "Import Oslo's common rootwrap to Neutron"